1. Field of the Invention
The present invention relates to an identification device for identifying the type of a recording medium.
2. Description of the Related Art
Image forming apparatuses such as copiers and laser printers include a latent image bearing member for bearing a latent image thereon, a developer for developing the latent image into a visible toner image, a transfer unit for transferring the toner image onto a recording medium, and a fixing unit for fixing the toner image transferred onto the recording medium. The fixing unit heats and compresses a material to be recorded under predetermined fixing conditions (such as a fixing temperature and a conveying speed at which the recording medium passes through the fixing unit, which are determined in advance depending on the paper type and the like) to fix the toner image onto the recording medium.
Recently, an image forming apparatus including an identification device configured to capture a surface image of a size or type of a recording medium into an operation panel or the like and to identify the type of the recording media, such as glossy paper, plain paper, rough paper, or overhead transparency (OHT) sheet, from a read result (image data) of the surface image has been proposed (see Japanese Patent Laid-Open No. 2002-182518). Such an image forming apparatus sets optimum image forming conditions including a printing density, a transfer bias (voltage value to be applied to the transfer unit), a fixing temperature, and a process speed (image forming speed) on the basis of an identification result of the identification device.
The identification device focuses light reflected from the recording medium onto an image pickup element such as a charge-coupled device (CCD) sensor or a complementary metal oxide semiconductor (CMOS) sensor through a lens, and performs analog-to-digital (A/D) conversion of an analog signal output from the image pickup element to obtain an image of a surface of the recording medium. The identification device identifies the type of the recording medium by, for example, calculating the depth of irregularities in the surface of the recording medium from the image (see Japanese Patent Laid-Open No. 2003-302208). More specifically, the identification device determines a peak-peak value, which is a difference between a maximum light intensity value and a minimum light intensity value for each row in the image of the surface of the recording medium. The determined peak-peak value is compared with a plurality of predetermined threshold values to determine the state of the surface of the recording medium to thereby identify the type of the recording medium, such as glossy paper, plain paper, rough paper, or OHT sheet.
In such existing identification devices, however, if variations in light intensity are large due to variations in intensity of a light source adapted to irradiate the recording medium with light or in transmittance of a lens, variations occur in the peak-peak value determined from images obtained by photographing recording media of the same type. Variations in sensitivity of photoelectric conversion cells of an image pickup element may cause larger variations in the peak-peak value. That is, the variations in the peak-peak value due to the variations of the lens and image pickup element of the identification device might reduce identification accuracy for identifying the type of recording medium. To ensure high identification accuracy, there is a need to reduce variations in light intensity of the light source, variations in transmittance of the lens, and variations in sensitivity of the photoelectric conversion cells. However, if the lens and the image pickup element are manufactured so as to reduce the variations in the light intensity, the transmittance, and the sensitivity, the cost increases.
Moreover, nonuniformity of the intensity of light emitted from the light source causes nonuniform light intensities within an irradiated area to change the peak-peak value. It is therefore difficult to accurately identify the type of the recording medium. To eliminate or reduce the nonuniformity of the light intensity of the light source within the irradiated area, there is a need for a correction operation called shading correction. Shading correction is a correction method in which an image of a surface of a recording medium is captured a plurality of times with the movement of the recording medium, a difference value between an average of light intensity output values of all pixels within the captured image areas of the image and a light intensity output value of each of the pixels is determined, and the determined difference values are added to the light intensity output values of the corresponding pixels within the image of the surface of the recording medium when the type of the recording medium is identified. In the implementation of shading correction, therefore, a plurality of images of a surface of a recording medium are captured and a correction operation is performed using the values obtained by capturing the images, which requires a long time to identify the type of the recording medium.